In general, lighting devices can be used for a multiplicity of applications in which the generation of light is required. By way of example, lighting devices are used for displaying information, e.g. in displays, in advertising panels or in mobile radio devices, and/or for illuminating objects or spaces, e.g. in the form of flat illumination modules. Such lighting devices can be based on the principle of electroluminescence, which makes it possible to convert electrical energy into light with high efficiency. By way of example, said lighting devices may include semiconducting materials, e.g. in the form of organic light emitting diodes (OLEDs) or inorganic light emitting diodes (LEDs), which make it possible to generate and emit colored light in the form of patterns or with a specific color valence.
In the switched-off state, however, these lighting devices reflect externally generated light with a color valence that is determined by the design of the lighting devices. By adapting said color valence, it is possible for lighting devices in the switched-off state, e.g. during the day or when they are not in use, to be integrated into their environment particularly well, that is to say that they are particularly inconspicuous. This so-called off-state appearance (OSA) of the lighting devices is a major factor influencing economic aspects, such as the sales of lighting devices.
In order to influence the OSA, color filters, e.g. in the form of color filter films, are conventionally applied to lighting devices, such that they determine the OSA of the lighting devices. However, the generated light of the light source in the switched-on state of the lighting device is altered by the color filter since the latter absorbs part of the light. Therefore, the emission color of the light source has to be correspondingly adapted to the color filter in order to maintain a desired emission color in the switched-on state. In other words, the color filter dominates the OSA of the lighting device, or the light source thereof together with the OSA forms the desired color impression. However, such color filters lead to a loss of efficiency as a result of the light absorption and can furthermore be used only to a limited extent, e.g. if colored patterns are intended to be generated.
Alternatively, the OSA can be influenced by conversion layers which convert the wavelength of part of the light into a different wavelength and naturally have the desired color. By way of example, conversion layers including scattering conversion centers convert the emission characteristic of the surface light source into a Lambertian emitter. However, only a very restricted material selection is available for conversion layers and this likewise brings about a loss of efficiency as a result of light scattering.
These conventional solutions are static and, besides the OSA, likewise influence the color valence of the lighting device in the switched-on state.